Toggling Stereochemical Activity through Interstitial Positioning of Cations between 2D V2O5 Double Layers

The 5/6s2 lone-pair electrons of p-block cations in their lower oxidation states are a versatile electronic and geometric structure motif that can underpin lattice anharmonicity and often engender electronic and structural instabilities that underpin the function of active elements in nonlinear optics, thermochromics, thermoelectrics, neuromorphic computing, and photocatalysis. In contrast to periodic solids where lone-pair-bearing cations are part of the structural framework, installing lone-pair-bearing cations in the interstitial sites of intercalation hosts provides a means of a systematically modulating electronic structure through the choice of the group and the period of the inserted cation while preserving the overall framework connectivity. The extent of stereochemical activity and the energy positioning of lone-pair-derived mid-gap states depend on the cation identity, stoichiometry, and strength of anion hybridization. V2O5 polymorphs are versatile insertion hosts that can accommodate a broad range of s-, p-, and d-block cations. However, the insertion of lone-pair-bearing cations remains largely underexplored. In this article, we examine the implications of varying the 6s2 cations situated in interlayer sites between condensed [V4O10]n double layers. Systematic modulations of lattice distortions, electronic structure, and magnetic ordering are observed with increasing strength of stereochemical activity from group 12 to group 14 cations. We compare and contrast p-block-layered MxV2O5 (M = Hg, Tl, and Pb) compounds and map the significance of local off-centering arising from the stereochemical activity of lone-pair cations to the emergence of filled antibonding lone-pair 6s2–O 2p-hybridized mid-gap states mediated by second-order Jahn–Teller distortions. Crystallographic studies of cation coordination environments and the resulting modulation of V–V interactions have been used in conjunction with variable-energy hard X-ray photoelectron spectroscopy measurements, first-principles electronic structure calculations, and crystal orbital Hamilton population analyses to decipher the origins of stereochemical activity. Magnetic susceptibility measurements reveal antiferromagnetic signatures for all the three compounds. However, the differences in V–V interactions significantly affect the energy balance of the superexchange interactions, resulting in an ordering temperature of 160 and 260 K for Hg0.5V2O5 and δ-Tl0.5V2O5, respectively, as compared to 7 K for δ-Pb0.5V2O5. In δ-Pb0.5V2O5, the strong stereochemical activity of electron lone pairs and the resulting electrostatic repulsions enforce superlattice ordering, which strongly modifies the electronic localization patterns along the [V4O10] slabs, resulting in disrupted magnetic ordering and an anomalously low ordering temperature. The results demonstrate a versatile strategy for toggling the stereochemical activity of electron lone pairs to modify the electronic structure near the Fermi level and to mediate superexchange interactions.

Table S1.Refinement statistics and structural details for the structure of Hg x V 2 O 5 shown in Fig. 1B S4.Bond distances and bond angles deduced for the δ-Pb x V 2 O 5 structure (Triclinic, P-1)

Figure S1 .
Figure S1.Electron microscopy characterization of δ-Tl x V 2 O 5 and Hg x V 2 O 5 .(A) Scanning electron microscopy (SEM) image of Hg x V 2 O 5 particles with lengths spanning hundreds of micrometers.(B) Energy dispersive X-ray spectroscopy (EDS) map of the elemental distributions of Hg, V, and O. (C) SEM image of δ-Tl x V 2 O 5 nanowires.(D) EDS integrated across δ-Tl x V 2 O 5 nanowires shown in (C).(E) Low-magnification transmission electron microscopy (TEM) image of δ-Tl x V 2 O 5 nanowires showing an average width of 162±88 nm.(F) Lattice-resolved TEM image with selected area electron diffraction (SAED) pattern indexed to the C 2/m cell derived from Rietveld refinement of the powder XRD pattern.

Figure S2 .
Figure S2.Electron microscopy characterization of δ-Pb x V 2 O 5 .(A) SEM image collected for a large δ-Pb x V 2 O 5 crystal and (B) corresponding EDS spectrum corroborating the assigned stoichiometry of δ-Pb 0.5 V 2 O 5 .

Figure S3 .
Figure S3.Density of States as calculated from ground state electronic structure calculations.(A) Total and atom-projected density of states for Hg 0.5 V 2 O 5 .(B) Total and atom-projected density of states for δ-Tl 0.5 V 2 O 5 (C) Total and atom-projected density of states for δ-Pb 0.5 V 2 O 5 .(D) Orbital-projected density of states for Hg 5d and 6s orbitals in Hg 0.5 V 2 O 5 .(E) Orbital-projected density of states for Tl in δ-Tl 0.5 V 2 O 5 .(F) Orbital projected density of states for Pb in δ-Pb 0.5 V 2 O 5 .

Figure S4 .
Figure S4.Magnetic susceptibility measurements of Hg 0.5 V 2 O 5 at varying field strengths.Temperature dependence of magnetic susceptibility of Hg 0.5 V 2 O 5 between 2−400K at an applied field of (A) 0.1 T, (B) 1 T, (C) 3 T, and (D) 5 T.

Figure S6 .
Figure S6.Inverse magnetic susceptibility of Hg x V 2 O 5 and δ-Tl x V 2 O 5 .Reciprocal plot of the magnetic susceptibility data at 2−400 K and 300-700 K and results of the Curie−Weiss fitting for (A) Hg 0.5 V 2 O 5 and (B) δ-Tl 0.5 V 2 O 5 , respectively.(For δ-Tl 0.5 V 2 O 5 , the compound exhibits a nonlinear dependence of inverse susceptibility at low temperatures (between 280-400 K), making accurate determination of valid Weiss constants or effective moments difficult.As such, we collected data well above the transition temperature (400-700 K)).